The present invention relates to the field of antennas, and more specifically to an antenna wherein the position of the phase center of the antenna relative to the primary radiating/receiving element is electronically controllable. The antenna is particularly useful as a tracking antenna feed.
Tracking antennas generally utilize special types of antenna feeds onto which the received electromagnetic energy is focused by highly directional lens or reflector systems. These feeds, known as tracking feeds, are structured and operated so that the received signal is modulated in phase and amplitude as a function of the position of the signal source relative to the boresight of the antenna. A tracking control circuit derives signal source position information from the received signal and points the antenna in accordance with this information.
Several different types of tracking feeds are commonly employed in this type of system. In a conical scan system, the main receiving element is located slightly off the vertex of the lens or reflector, and is physically rotated (scanned) about the vertex. As long as the signal source is located off the boresight of the antenna, the received signal will be modulated in accordance with this scanning action. Tracking information can thus be derived from the received signal.
In pseudomonopulse tracking feeds, a number of secondary receiving elements (usually four) are provided in addition to the main receiving element. The main receiving element is located at the vertex of the reflector (or lens), and the secondary elements are arrayed about it. The signals from these secondary elements are sequentially combined with the signal from the main receiving element, located at the vertex, in such a manner that again a modulated signal is derived from which tracking information can be obtained. The operation of this type of system is described more fully in the detailed description which follows.
Pseudomonopulse tracking feeds have the advantage that, unlike conical scanning tracking feeds, the feed includes no moving parts. Pseudomonopulses tracking feeds do, however, usually require at least two hybrids for signal comparison purposes, as well as a directional coupler and a scanning circuit. The resulting tracking feeds operates quite well, but is somewhat large and bulky.